[Problem] To provide a novel fluorescent probe that has superior tissue permeability and is capable of detecting the activity of peptidase highly expressed in cancer cells using a fluorescent response having a longer wavelength. [Solution] This fluorescent probe for detecting peptidase activity includes a compound represented by formula (I) or a salt thereof [in the formula: X represents Si(Ra)(Rb) (where Ra and Rb each independently represent a hydrogen atom or an alkyl group)； Y represents a C0-C3 alkylene group, and the alkylene group may be substituted with a halogen atom or haloalkyl； R1 represents 1-4 substituents, which are the same or different from each other, and which are independently selected from the group consisting of a hydrogen atom, a cyano group, and optionally substituted alkyl group, carboxyl group, ester group, alkoxy group, amide group, and azide group； R2 forms, together with R3, a 5- to 8-membered heterocyclic structure containing a nitrogen atom and a carbon atom to which R2 and R3 are bound； R4 forms, together with R5, a 5- to 8-membered heterocyclic structure containing a nitrogen atom and a carbon atom to which R4 and R5 are bound； R6, R7, R8, and R9 each independently represent a hydrogen atom, a halogen atom, a hydroxyl group, an alkyl group, a sulfo group, a carboxyl group, an ester group, an amide group, or an azide group； and R10 represents an amino acid-derived acyl residue].Le problème décrit par la présente invention est de fournir une nouvelle sonde fluorescente qui présente une perméabilité tissulaire supérieure et qui est capable de détecter l'activité de peptidases hautement exprimées dans les cellules cancéreuses à l'aide d'une réponse fluorescente ayant une longueur d'onde supérieure. La solution selon l'invention porte sur une sonde fluorescente pour détecter une activité peptidase comprenant un composé représenté par la formule (I) ou un sel de celui-ci [dans la formule : X représente Si(Ra)(Rb) (dans lequel Ra et Rb repr
